In heat exchangers employing tubes arranged in a bundle, it is necessary to maintain the individual tubes of the bundle in a parallel, spaced relationship. Such spacing is important to allow the even transfer of heat from fluids flowing within the tubes to fluids flowing around the outside of the tubes. There are advantages to employing heat exchangers as gas turbine regenerators, vapor generators, feed water heaters, and the like, in which a first fluid is passed through the tubes and a second fluid is passed in counterflow relation to the first fluid around the outside of the tubes along the length thereof. In heat exchangers of this type, it is also necessary for the support structure maintaining the tubes in spaced, parallel relationship to be open to permit fluid to flow by.
U.S. Pat. No. 3,916,990, issued Nov. 4, 1975 to Anthony Ruhe et al and assigned to the assignee of the present application, discloses a heat exchanger in which a plurality of elongated tubes are supported in a spaced parallel relationship between two tube sheets, and in which intermediate support units are provided between the tube sheets, the support units comprising a plurality of interconnected grid strips formed into a grid pattern for permitting fluid to flow through. The support units are preassembled in a shop by welding the strips together at various spaced points. Such preassembled support units or grids are well suited for straight lengths of tubing, for the tubes can be slid through the support units and into the tube sheets during the assembly of the heat exchanger.
Known tube-type heat exchangers for the above-mentioned purposes usually include a bundle of straight tubes extending between inlet and outlet headers at opposite ends of a shell or casing. A drawback of this type of heat exchanger is the tendency of some tubes to develop high thermal stresses as a result of relative expansion compared to other tubes in the bundle. This is attributable to the lack of uniform gas flow across the width of the bundle. A type of tube heat exchanger which overcomes this problem includes an elongated bundle of U-shaped tubes mounted within a large U-shaped shell or casing. In addition to avoiding the tube stressing mentioned above, the U-shaped heat exchanger is characterized by a reduction in required materials and construction costs. For example, fewer tube-to-tube sheet joints are associated with such a heat exchanger than would be required for a conventional tube-type heat exchanger of equivalent capacity. However, one requirement of such U-tube heat exchangers which does not apply to other heat exchangers is the need for intermediate supports in the area of the U-bend to prevent vibration of the tubes and to maintain the tube bundle in a properly spaced orientation. Furthermore, where the flow outside the tubes is parallel to the tubes, the parallel flow must be maintained in the U-bend area as well. This imposes on the intermediate U-tube supports the further requirement of high porosity to enable fluid to flow through the support structure with very low flow resistance.
The previously mentioned preassembled grids are not suitable for the area of the U-bend because the tubes cannot be slid with respect to preassembled grids where the tubes bend. Thus, it also required that the intermediate supports in the area of the U-bend can be built up layer-by-layer as the tube bundle is built up.